These studies will continue our investigation of the genetics of human proteins using cell culture techniques. We will continue to study the mapping of the enzyme galactose-1-phosphate uridylyltransferase and UDP galactose epimerase in human-mouse hybrids. In addition further research will be done to analyze the effect of an inhibitor N-bromoacetyl beta-D-galactosylamine on the turnover of cell proteins. This inhibitor seems to offer a unique system which will be extremely valuable in studying the rapidly turning over proteins in cultured human cells. The effect of NBAG is reversible and can now be used in conjunction with radioactive pulsing techniques and enzyme analysis to characterize which proteins are indeed rapidly turning over in these cells. Revertants at the locus for galactose-1-phosphate uridylytransferase have now been isolated from SV-40 transformed human galactosemic cell lines. These variants are now being characterized to demonstrate the nature of the variations in the structural protein on reversion from the galactosemic mutation to an enzyme which is functionally active in cells. The amount of cross-reacting material to GALT antibody will be measured in these cells so that their actual specific activity will be determined. Alterations in the charge of the molecule will be studied by isoelectric focussing. Cells which have reverted to the ability to grow in hexose free medium will also be studied to determine what the energy souce of the cells are under these conditions. Mutation rates at the GALT locus will be measured using this approach.